I did BDA on a couple of occasions and, even with modern technology, it can be challenging. Having looked at some of the WW2 imagery, I'm astounded at the results the Photo Interpreters achieved.
Area bombing did simplify the problem somewhat. The British simply estimated the acreage of a target city devastated and didn't worry to much about specific targets within the devastated area. Even this could be difficult. The British post raids report on Hamburg noted that smoke obscured much of the evidence of destruction, but that the "amount of residential damage is very great."
The Germans agreed. The cumulative damage caused by Operation Gomorrah was estimated by the local authorities to be the destruction or damaging of 61% of Hamburg's houses and apartments. Similarly, 580 business premises and 2,632 shops were damaged or destroyed. 900,000 people evacuated the city and had to be looked after. The initial figure for those killed was 31,647, later revised to 38,975. The British didn't realise just how successful (in their terms) the raids had been.
Cheers
Steve